Technical Field
Embodiments of the present disclosure relate to a method for compensating instantaneous power failure in a medium voltage inverter and, more particularly, to a method for compensating instantaneous power failure for a cascaded H-bridge (CHB) type high-capacity medium voltage inverter and a voltage inverter system using the same.
Description of the Related Art
In general, when input power failure occurs, an inverter interrupts pulse width modulation (PWM) output within a few milliseconds. In this case, a long time is taken for a load having high inertia such as a motor to restore power as the load is accelerated. If a long time is taken to recover power, this may be a great loss for industries. Accordingly, a technology for compensating instantaneous power failure in the inverter is applied in places where stopping the inverter can result in considerable damage.
FIG. 1 is a graph illustrating a conventional method for compensating instantaneous power failure.
As shown in FIG. 1, when the input voltage is reduced to a voltage less than or equal to a first threshold voltage at t1, the output frequency is reduced by a predetermined value. Thereafter, the output frequency is reduced at a predetermined deceleration rate until t2, which is the restoration time. By reducing the output frequency as described above, regenerative energy for controlling a power failure period at the initial stage of power failure may be obtained. According to the conventional method for compensating instantaneous power failure described above, the power failure period is coped with by converting kinetic energy stored in a load into electrical energy. Thereby instantaneous power failure lasting for more than 16 ms may be coped with, which is not possible with the conventional medium voltage inverter employing the CHB technique.
However, the conventional technology for compensating instantaneous power failure in an inverter as illustrated in FIG. 1 has the following problems.
First, according to the conventional technology for compensating instantaneous power failure in an inverter, regenerative energy from the motor and power are simultaneously supplied at the power restoration time t2. Thereby, the voltage of a DC link increases, resulting in overvoltage trip.
Second, the actual speed of the motor does not vary linearly with the motor capacity or load. There is, if the slip frequency increases beyond a certain value due to nonlinearity of the motor speed, output current increases, thereby resulting in overcurrent trip.